Remove unnecessary migration deletion

Since isolation application is generated with the `--skip-gemfile`
option, so `active_storage:install` is not executed.

railties/test/application/bin_setup_test.rb

@@ -16,8 +16,6 @@ def teardown
 
     def test_bin_setup
       Dir.chdir(app_path) do
-        FileUtils.rm_rf("db/migrate")
-
         app_file "db/schema.rb", <<-RUBY
           ActiveRecord::Schema.define(version: 20140423102712) do
             create_table(:articles) {}
@@ -39,8 +37,6 @@ def test_bin_setup
 
     def test_bin_setup_output
       Dir.chdir(app_path) do
-        FileUtils.rm_rf("db/migrate")
-
         app_file "db/schema.rb", ""
 
         output = `bin/setup 2>&1`

railties/test/application/rackup_test.rb

@@ -26,7 +26,6 @@ def teardown
 
     test "config.ru can be racked up" do
       Dir.chdir app_path do
-        FileUtils.rm_rf("db/migrate")
         @app = rackup
         assert_welcome get("/")
       end

railties/test/application/rake/dbs_test.rb

@@ -287,8 +287,6 @@ def db_test_load_structure
           ENV.delete "RAILS_ENV"
           ENV.delete "RACK_ENV"
 
-          Dir.chdir(app_path) { FileUtils.rm_rf("db/migrate") }
-
           app_file "db/schema.rb", <<-RUBY
             ActiveRecord::Schema.define(version: "1") do
               create_table :users do |t|
@@ -310,8 +308,6 @@ def db_test_load_structure
       end
 
       test "db:setup sets ar_internal_metadata" do
-        Dir.chdir(app_path) { FileUtils.rm_rf("db/migrate") }
-
         app_file "db/schema.rb", ""
         rails "db:setup"
 

railties/test/application/test_runner_test.rb

@@ -11,7 +11,6 @@ class TestRunnerTest < ActiveSupport::TestCase
     def setup
       build_app
       create_schema
-      remove_migrations
     end
 
     def teardown
@@ -728,10 +727,6 @@ def create_schema
         app_file "db/schema.rb", ""
       end
 
-      def remove_migrations
-        Dir.chdir(app_path) { FileUtils.rm_rf("db/migrate") }
-      end
-
       def create_test_file(path = :unit, name = "test", pass: true)
         app_file "test/#{path}/#{name}_test.rb", <<-RUBY
           require 'test_helper'

railties/test/application/test_test.rb

@@ -8,7 +8,6 @@ class TestTest < ActiveSupport::TestCase
 
     def setup
       build_app
-      remove_migrations
     end
 
     def teardown
@@ -321,10 +320,6 @@ class UserTest < ActiveSupport::TestCase
     end
 
     private
-      def remove_migrations
-        Dir.chdir(app_path) { FileUtils.rm_rf("db/migrate") }
-      end
-
       def assert_unsuccessful_run(name, message)
         result = run_test_file(name)
         assert_not_equal 0, $?.to_i